A Labour Europe spokesman has said the UK should not permanently rule out joining the euro and called for the party to adopt a less "defensive" stance on the EU.

Lord Liddle told The Guardian it was "just ludicrous" for politicians to suggest Britain could cut itself off from the single currency altogether in case it emerged stronger from its present crisis.

Shadow chancellor Ed Balls declared in 2011 that there was " no possibility of the euro being joined by a British government at any time in my lifetime".

As a Treasury adviser to Gordon Brown during the Labour administration, Mr Balls was instrumental in keeping the UK out of the single currency.

Speaking on the eve of talks between Labour leader Ed Miliband and German chancellor Angela Merkel during her visit to the UK, Lord Liddle suggested membership remained possible in 10 to 20 years.

The former adviser to Tony Blair and the EU Commission president told the newspaper: "The attitude of mind of people that we can cut ourselves off from this is just ludicrous.

"We are an integral part of an integrated European economy. So, in time, if the euro sorts itself out surely the logic is: why should we hang back from that?"

"If the euro does overcome its problems it is arguable that Britain has a flexible enough economy to be able to survive within the euro's constraints.

"We are now more convergent with the euro area than we have ever been.

"One shouldn't close one's mind on ideological grounds to the possibility of joining."

Labour, he said, had " failed... to integrate Europe into its whole thinking about what it is trying to do".

Mr Miliband faces pressure from within the shadow cabinet to match David Cameron's promise of an in/out referendum on the UK's membership of the EU.

"Labour is defensive about Europe.

"The core of the case that Labour makes for our membership of the EU is made on jobs and growth.

"We make the case strongly. But we could do much more to demonstrate how European solutions are complementary to what we want to do at home in tackling all these challenges we face."

In his book, The Europe Dilemma: Britain And The Drama Of EU Integration, the peer argues that if a stronger version of the euro emerges, " progressives should not be content with Britain's unsatisfactory status quo as a euro-out where we have little influence on many key economic decisions impacting on Britain's future".

A Labour spokesman said: "Labour in government made the right decision not to join the euro a decade ago. It's a decision which has stood the test of time.

"Ed Miliband and Ed Balls have both said that joining the euro would not happen in their political lifetime. It will not happen."